In this paper, a method for evaluation of power frequency estimation during transient operation in distributed generation (DG) in islanding conditions is presented. For this end, five methods of fundamental frequency estimation are evaluated in a distribution system with distributed generation in islanding operation, such as: (a) zero crossing, (b) adjustment of points to a pure sinusoidal waveform, (c) hybrid method, (d) discrete Fourier transform method based, and (e) Prony-Fourier-based algorithm. The estimators are compared and evaluated in two different ways, using two analytical functions, and in an electric power distributed system with DG in islanding condition. The evaluation methodology for the frequency estimators are based in statistical errors, taking into account mean, mean square error. The zero-crossing and discrete Fourier transform based estimator presented better results in comparison with the other evaluated algorithms. However, the discrete Fourier transform presented good results in the estimated frequency in the three evaluated cases.
Słowa kluczowe: Frequency estimation, fundamental frequency, distributed generation, islanding condition, protection.
Streszczenie. W artykule oceniono metody badania częstotliwości w stanach przejściowych rozproszonych sieci energetycznych w warunkach tworzenia się wysp. Zbadano pięć metod: zero crossing, APPSW, hybrydową, DFT iProny. Najlepszy wyniki osiągnięto przy stosowaniu metody zero-crossing (Przeciecia osi zero) I DFT. Ocena metod określania częstotliwości w stanach przejściowych rozproszonych sieci energetycznych w warunkach tworzenia się wysp.
Keywords: określanie częstotliwości, rozproszone sieci energetyczne, powstawanie wysp
The power system frequency has an important role in order to perform monitoring and controlling the power quality in electrical distribution systems with distributed generation (DG). However, in electrical systems with high penetration of DG changes occurs such as the unintentional islanding of DG for example. Islanding situations can occur with the disconnection of DG during the normal operation, then, the frequency, changes suffer from the high load disconnection of the system that can affect the safety and reliability of the islanded area. For this reason, many algorithms are presented for fundamental frequency estimation applied in electric power systems, such as: zero-crossing method [1, 3], adjustment of points to a pure sinusoidal waveform (APPSW) , hybrid method [4, 5], discrete Fourier transform method (DFT) [2, 6], Prony-Fourier-based algorithm [7, 8], and other methodologies such as: Kalman filtering , least mean square (LMS) . To solve the problem of islanding detection, schemes are used to detect an islanding operation of DG. Different methods for islanding detection have been reported in recent years [14, 15]. Islanding detection techniques are usually divided into: (a) local detection methods when the detection is based on the DG side, and (b) remote detection methods when the detection is based on the network side. Local detection methods can be passive methods or active methods . The passive methods monitor voltages and currents waveforms. Voltage, frequency, harmonic and phase measurements are taken separately or simultaneously to determine an islanding situation. Frequency-based schemes [15, 16] as frequency relay [14-16], rate of change of frequency (ROCOF) relay [17, 18] were studied to islanding detection. However, in the aforementioned papers, the transient state is not taken in account in the frequency estimation. The transient state affects the frequency estimation quality. In tran [...]
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